201 research outputs found
Steady current induced vibration of near-bed piggyback pipelines: Configuration effects on VIV suppression
No full textA series of experiments on steady current induced vibration of piggyback pipelines close to a plane seabed were conducted with a hydro-elastic facility in a conventional water flume. The effects of the mass-damping parameter, the diameter ratio, the gap-to-diameter ratio, the spacing-to-diameter ratio and the position angle on the VIV response were studied. The VIV suppression for the piggyback pipeline system by the small pipe was investigated based on the analysis of the vibration amplitude and the critical reduced velocity for the onset of VIV. Comparison with the prediction with the modified Griffin plot by Govardhan and Williamson (2006) [10] shows that the peak vibration amplitude of near-wall piggyback pipelines is smaller than that for a wall-free single pipe. The configuration parameters of piggyback pipelines have significant effects on the VIV suppression. For the configuration of the small pipe above the main pipe (theta = 90 degrees), the minimum peak amplitude and the maximum critical reduced velocity occur at the spacing-to-diameter ratio of G/D approximate to 0.25, indicating that VIV is suppressed most effectively by the small pipe at this value of G/D. For a constant value of G/D = 0.25, both the minimum peak amplitude and the maximum critical reduced velocity occur at the position angle of theta approximate to 120 degrees. (C) 2014 Elsevier Ltd. All rights reserved
High Aspect Ratio (L/D) Riser Viv Prediction Using Wake Oscillator Model
No full textA two-dimensional (2-D) vortex-induced vibration (VIV) prediction model for high aspect ratio (LID) riser subjected to uniform and sheared flow is studied in this paper. The nonlinear structure equations are considered. The near wake dynamics describing the fluctuating nature of vortex shedding is modeled using classical van der Pol equation. A new approach was applied to calibrate the empirical parameters in the wake oscillator model. Compared the predicted results with the experimental data and computational fluid dynamic (CFD) results. Good agreements are observed. It can be concluded that the present model can be used as simple computational tool in predicting some aspects of VIV of long flexible structures. (C) 2008 Elsevier Ltd. All rights reserved
VIV analysis of a subsea power cable during installation in high current seas
No full textTideway is a marine solutions provider for the offshore oil & gas and renewables and among other activities specialized in subsea power cable installation. To optimize the installation of power cables in high currents a better understanding of the dynamic behavior of the cables is needed. During installation of subsea power cables in high ocean currents, significant vibrations might occur. These vibrations are called vortex induced vibrations (VIV) and they are result of the vortex shedding behind the cables. Significant forces may arise due to VIV which can compromise the cable's integrity limits and further introduce problems to the installation procedure. The main objective of this research is to assess the effects of the VIV on the cables and provide solutions to improve workability. In this thesis a coupled wake-structure model was created in order to study the interaction between the vortices and the cable. The model is based on the phenomenological model of the wake oscillator which can capture well the self-exciting, self-limiting behavior of VIV, therefore estimating accurately enough the lock-in region and vibration amplitude. A wake oscillator with acceleration coupling is used and different sets of tuning parameters tuned to 1DOF and 2DOF free vibration experiments are examined. The structure has been modeled in the finite element software OrcaFlex and the wake oscillator model was implemented as an external function in the programming language Python. To confirm the validity of the above mentioned model two experiments were used. Firstly, the model was tested against the Delft flume experiment where the case of a vertical riser is examined and further the São Paulo experiment where the case of a steel catenary riser (SCR) is studied. The model was also compared with the Milan wake oscillator model from OrcaFlex's VIV toolbox. The model showed good agreement with the experimental results and found to be superior of OrcaFlex's VIV model. Next, VIV analysis was performed for a case study. The analysis was done for different current speeds and directions and the effects of the VIV on the tension and the bending radius of the cable were determined. For lower current velocities, lock-in response and standing waves along the cable were observed, while for higher velocities multifrequency response, beating behavior and traveling waves were observed. An important conclusion of this research is that the coupled wake-structure model used in this thesis is suitable for assessing VIV effects on catenary shaped cables. Furthermore, the analysis showed that catenary shaped cables with remaining length on the seabed exhibit smaller crossflow vibration amplitudes than straight cylinders
The influence of peri-conception and first trimester dietary restriction of protein in cattle on meat quality traits of entire male progeny
No full textAbstract not availableTharcilla I.R.C. Alvarenga, Katrina J. Copping, Xuemei Han, Edward H. Clayton, Richard J. Meyer, Raymond J. Rodgers, I. Caroline McMillen, Viv E.A. Perry, Geert Geesin
Over het voorkomen van Trifolium micranthum Viv. in Nederland
No full textThe author gives an enumeration of the localities in which Trifolium micranthum Viv. is found in the Netherlands
EFFECTS OF TOP-END VESSEL HEAVE ON SUBMARINE RISER VIV OF DEEP WATER PLATFORM
No full textThe dynamic coupling between moving top-end vessel and submarine riser becomes more remarkable for a floating platform in deeper water due to the larger top-end motion amplitude, compared with the fixed platform in shallow water. In this study the impacts of top-end heave on the riser undergoing vortex-induced vibration (VIV) are explored in terms of the parametric excitation and the consequent dynamic behaviors. By using finite element simulations based on a coupled hydrodynamic force approach, the dynamic responses of the integrated system including both a floating top-end and the riser experiencing VIV are examined. Our numerical results show that the riser displacement becomes several times larger than the displacement for the case without top-end motion, and the impact of heave on riser VIV response gets larger as the modal order number dropping. Riser VIV amplitude becomes, almost linearly, more profound when the tension ratio, as one of critical parameters that influence the riser dynamic response, gets larger. Moreover, an interesting phenomenon called mode transition is observed, particularly at lower frequency, during modal dynamics response
On the measurement of VIV lift force coefficients at high Reynolds numbers
No full textThe objective of the research was the measurement of the VIV lift force coefficient in-phase with velocity Clv and in-phase with acceleration Cla for a pipe section with large length over diameter ratio of L/D ~18 at high Reynolds numbers of Re > 1E4. The coefficients are measured for a forced oscillation pipe in a steady flow and can be directly used as input parameter for pragmatic riser VIV prediction models. Risers are vertical pipelines that transport fluids from the oil well on the seabed to the production facility in the free water surface. The risers in deep water are extremely slender structures, having length over diameter ratio of more than L/D = 1E3. The risers in deep water behave as a flexible string-like structure with low structural damping, which makes them susceptible for resonant vibrations. The vibrations caused by the vortex shedding in the downstream wake of the riser are known as Vortex Induced vibration (VIV) and occur when the frequency of the vortex shedding coincides with one or more of the natural frequencies of the riser. The VIV of the riser poses large challenges for the design of the risers, in particular related to metal fatigue.Ship Hydromechanics and Structure
DYNAMIC COUPLING BETWEEN TOP-END VESSEL SWAY AND SLENDER RISER VIV IN DEEP WATER
No full textThe impacts of top-end motion on the riser undergoing vortex-induced vibration (VIV) are explored in this study, because the dynamic coupling between moving top-end vessel and submarine riser becomes more remarkable for a floating platform in deeper water due to the larger top-end motion amplitude, compared with the fixed platform in shallow water. A coupled hydrodynamic force approach, involving the vortex-induced lift force along with the fluid drag force, is developed. The dynamic responses of the system including a floating top-end and a riser experiencing VIV are examined by means of finite element simulations. The effects of amplitude and frequency of top-end vessel sway on riser WV are examined. Our numerical results show that the riser displacement becomes several times larger than the displacement for the case without top-end motion. Moreover, the nonlinear response amplification is observed, and the nonlinear amplification gets more pronounced as the number of mode order dropping, while the amplification factor just slightly changes with the increase of sway amplitude
Vortex-induced vibrations in OTEC: An analysis of VIV of a cold water pipe for OTEC plants
No full textAs climate change seems to become more and more of a serious issue, a sudden surge of interest in clean and renewable energy sources is inevitable. Ocean Thermal Energy Conversion (OTEC), or the process of harnessing energy from the temperature difference between the warm upper ocean layer and the cold bottom ocean layer, is one of these renewable energy production methods. In order to pump up this cold water, a large diameter pipe of a length in the order of 1000 m is deployed. A pipe of such large properties will indubitably suffer from problems related to production, installation and drag forces. Up till now these problems have all largely been evaluated. One phenomenon that has been explored less frequently is VIV in OTEC, and this is therefore the main scope of this research.Vortex-induced vibrations (VIV) are the vibrations that occur when a current flows past a cylinder, shedding vortices at higher Reynolds numbers, leading to an asymmetrical pressure distribution. For this research, the pipe is modelled as a slender Euler-Bernoulli beam conveying fluid, with gravity and ambient flow considered. The model is discretised and rewritten into state-space representation, in order to make it suitable for Python’s odeint ordinary differential equation-solver. A wake oscillator model is introduced to model the VIV phenomenon, and the complete model is used to evaluate multiple scenarios to evaluate the influence of variables as pipe diameter, ballast mass and current velocity.A typical cold water pipe is made of a light material as for example HDPE, has a length in the order of 1000 m and a diameter of around 2.5 m, kept under tension by a ballast mass in the order of magnitude of 350 tonnes. For this research, this typical cold water pipe is taken as a starting point and subjected to a uniform current flow of 0.4 m/s. The VIV response of the pipe is then evaluated with a range of different variables. First, the pipe is evaluated in steel, FRP and HDPE to establish which material is most suitable. HDPE shows the best fatigue resistance, and is therefore chosen as the material in which the further research will be conducted. The pipe diameter is varied between 2.0 and 6.0 m, the diameter to wall thickness ratio between 12 and 26, the effect of the magnitude of the ballast mass is evaluated and the inflow velocity is changed between 1.0 and 6.0 m/s. Finally, the current flow is varied between a uniform current of 0.1 m/s and 0.8 m/s and a location specific sheared current profile is applied, in order to check whether VIV in the OTEC cold water pipe might occur in a realistic setting.Depending on the chosen variables, in general the pipe will experience a maximum cross-flow displacement per diameter ranging between 0.6 and 1.0. Furthermore, for each set of variables the pipe will vibrate in a different normal mode, which might lead to the fact that a pipe with less maximum displacement will still experience more fatigue damage. Pipes in larger diameters will definitely suffer from a higher fatigue, however the maximum experienced damage per year was in the order of 10-15, meaning that this is insignificant. The wall thickness will not have a lot of effect. The ballast mass however, can better be chosen high since it prevents vibrations at a high mode and it seems that HDPE is strong enough in terms of yield strength to survive a large mass. Finally, when the pipe is subdued to high currents, it seems as if in-line displacement will become a larger problem then VIV, as the in-line bending reaches dangerously high levels when subjected to a current of 0.8 m/s. It has to be noted however that certain measures can be taken against this displacement.It is very likely that VIV occurs in a cold water pipe of an OTEC plant as it is subjected to certain current flows, but in the case HDPE is used, this does not seem a large concern.Offshore and Dredging Engineerin
Motion Reconstruction of Vortex-Induced Vibration of Long Flexible Riser from Experimental and Field Test Data
No full textVortex-induced vibration (VIV) of long flexible cylindrical structures enduring ocean currents is ubiquitous in the offshore industry. Though significant effort has gone into understanding this complicated fluid-structure interaction problem, major challenges remain in modelling and predicting the response of such structures. The work presented in this thesis applies the modal approach to do motion reconstruction of the riser VIV from experimental data at first and then performs some analyses to the riser VIV response based on the reconstructed result. In the first part of the thesis, the modal approach is classified into the frequency domain method and the time domain method according to the types of the measurement data. Two systematic frameworks to do motion reconstruction are built for these two methods. Besides, two factors probably leading to the reconstruction error are proposed. One is using the strain measurement to identify the low modes VIV motion and the other one is unreasonable choice of participating modes. In the second part of the thesis, the riser VIV motion in ExxonMobil VIV test is reconstructed using the frequency domain method and that in the second Gulf Stream VIV test is reconstructed using the time domain method. In the reconstruction process, several problems are needed to be solved, such as the choice of time window, filtering data and the choice of participating modes. And the accuracy of the reconstructed result is verified using the extraction method. Finally, two examples are given to demonstrate the reconstruction errors induced by the above two facors. In the final part of the thesis, some key parameters are extracted out to show the effects of external conditions, e.g. current profile, current speed and strake coverage, on the VIV displacement magnitude and response frequency of the riser. Besides, three methods are provided to identify the travelling wave in the riser VIV response.Mechanical, Maritime and Materials EngineeringMarine and Transport TechnologyOffshore and Dredging Engineerin
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